The present invention relates to a receiver suitable for use in mobile communication systems.
Most of the receivers currently employed in mobile communication systems are based on the so-called superheterodyne system, which converts the frequency of a received signal into an intermediate frequency (IF). This is because it uses a high frequency band with a narrow channel spacing and permits comparatively easy realization of a detecting circuit. In an especially high frequency band, there is used the double superheterodyne systems, in which frequency conversion is performed twice. However, a receiver using the double superheterodyne system, as will be explained below, is subject to stringent performance requirements on the filter for suppression of image frequency signals and channel selection, and therefore tends to be very expensive.
One solution to this problem, is the direct conversion system described in the UK Patent No. 1,530,602 published in Nov. 1, 1978. This system, however, involves the problem of suppressing spurious radiation of local oscillator signals of the quadrature detector from the input antenna. This is because the setting of the frequency of the local oscillator signal must be substantially equal to that of received signals.